In a wireless environment, impairments of communication channels can affect significantly the performance of a wireless system. Multi-path fading is one of the most significant impairments.
Wireless communication systems such as CDMA, WCDMA, GSM/EDGE typically require accurate timing or synchronization with a base station and this is obtained from the estimation of the frequency of the received RF signal at a mobile device. In some wireless communication systems, the minimum level of accuracy of the frequency is 0.1 part per million (0.1 ppm). However in a typical design this will be about 0.04 ppm and this is achieved through the AFC (automatic frequency controller) system of the mobile device. The AFC system measures the frequency difference between the received RF signal and an onboard frequency and applies a correction in terms of a DC voltage offset to a correction feedback loop to keep the onboard frequency synchronized to the received signal. The onboard frequency may for example be based on a voltage controlled temperature compensated crystal oscillator (VCTCXO) that is relatively stable over a reasonable period of time, so that it can be used for the purpose of measuring Doppler effect to a certain level of accuracy. However, it is difficult or impossible to predict the exact Doppler shift (or frequency offset) at the mobile device for a given moment in time. This can be attributed to many factors such as multi-path (multiple routes a signal take from the base station to the mobile device), other fading effects due to environmental changes, the variability between base station line-of-sight and the direction of travel of the mobile device, etc.